There is provided a common method for establishing a clock synchronization between an OBS embedded in a satellite to conduct a multi-beam switching function and a satellite communication earth station referring to FIG. 1.
Differently from an existing communications satellite that simply converts and amplifies a frequency of an uplink signal and then converts an amplified frequency into a downlink signal for its relay, an OBS satellite that does multi-beam communication converts and amplifies a frequency of an uplink signal sent from each of earth stations 20 to 40, and also goes down a signal from each of the earth station 20 to 40 by beam-switching it to a desired area. By doing so, efficient use of limited satellite output is possible and frequency reuse is also allowable by minimizing interference between switched beams.
The satellite communication using such an OBS satellite is called SS-TDMA satellite communication, which is a scheme that transceives signals between areas divided into a plurality of beam regions over communication satellite that supports multi-beam communication, and improves output efficiency of satellite radio wave signal compared to existing satellite communications using single-beam communication satellite and further reuses frequency.
For this, however, the earth stations 20 to 40 prepared in each beam region must transmit a signal burst at the moment of switching operation of an OBS 10 embedded in the satellite. Upon failure, signal loss is occurred due to a discrepancy between the signal burst sent from the earth stations 20 to 40 to the OBS 10 embedded in the satellite and the switching operation of the OBS 10. In addition, signal loss is taken place if an operation clock of the OBS 10 embedded in the satellite is not precisely synchronized with that of the earth stations 20 to 40.
As described above, the switching operation of the OBS 10 embedded in the satellite should be accurately consistent with the signal transmission from the earth stations 20 to 40 for the SS-TDMA communication. Failure to establish the synchronization between the OBS 10 and the earth stations 20 to 40 causes a discontinuation of signal sent from the earth stations 20 to 40 to the satellite by the OBS 10, thereby rendering well preserved signals not transceived between the earth stations 20 to 40.
To solve the above problem, it needs to establish a precise synchronization between a Timing Source Oscillator (TSO) that creates a driving clock of a Digital Control Unit (DCU) to control switching operation of a Microwave Switching Matrix (MSM) embedded in the satellite and a Voltage Controlled Crystal Oscillator (VCXO) that produces a clock for signal transmission of an earth station.
One of prior arts for establishing a phase synchronization between TSO and VCXO is proposed in T. Inukai et al., “Onboard Clock Correction for SS/TDMA and Baseband Processing Satellites”, Comsat Technical Review, vol. 11, no. 1, pp. 77-100, Spring, 1981. This prior art discloses a method for conducting a phase synchronization by TSO embedded in a satellite by using VCXO of an earth station as reference.
According to the prior art, however, TSO is provided in the earth station whereas VCXO is prepared in the satellite, as opposed to the invention. Specifically, the earth station extracts a phase error or difference between its own TSO and a frequency of VCXO incorporated in the satellite by monitoring SS-TDMA signal from the satellite and using a start point and an end point of signal frame thereof, and then calculates a phase correction value. And then, it sends the correction value to the satellite over a Tracking, Telemetry and Command (TT&C) channel of a control center and VCXO embedded in the satellite adjusts the phase using the correct value.
However, the existing method has a drawback in that its hardware and software structure and algorithm are complicated to detect the phase error of VCXO in the satellite by the earth station and calculate the correction value.
In other words, in case where the clock synchronization between the OBS 10 in the satellite and the earth stations 20 to 40 is made based on the prior art, it requires a complicated hardware for extracting the phase error of the OBS 10 in the satellite, calculation algorithm of correcting a phase of TSO, and a series of works for sending commands to the satellite over the TT&C channel of the central center. This makes the hardware and software structure and algorithm complicated.